The present invention relates to moving target indicators for an airborne early warning radar, and more specifically, to the method and apparatus for reducing false target indications in such systems.
With the rapid advance in the development of weapons delivery systems, the early detection of airborne targets is increasingly critical to a successful defense. Efforts to increase the range of surface based radars have encountered numerous problems. For example, low flying aircraft are often shielded by the horizon from surface antennae in high frequency line-of-sight radars, and the physical size of the antenna and the enormous power supplies necessary for the transmission of low frequency radar energy are significant practical limitations. An additional problem has been the atmospheric trapping of the radar energy due to temperature inversion and other atmospheric conditions.
To avoid these problems, the prior art has resorted to the use of radar pickets or outposts spaced from the defended area. By using aircraft as the radar platform for these outposts, the problem of atmospheric trapping is reduced, and higher frequency signals requiring less power and physical antenna size can be utilized due to the effective increase in the height of the antenna with respect to the horizon. However, while the clutter of ground and sea return is a limiting factor only at ranges in the order of 5 to 10 miles for upward looking and outward looking surface based radars, airborne downward looking radars are severely limited by the clutter. This problem is particularly acute where the target is in close proximity to the ground.
In blue water environments, these airborne early warning or AEW aircraft, through the use of a comparatively large antenna, a rather low speed platform and a high performance clutter cancellation circuit have been successful in reducing the display of radar energy reflected from the sea to a point where the system is effective.
Additional problems, however, arise in the overland environment in that the ground return from an enormous number of stationary reflectors, buildings, cliffs, fences and the like, may completely saturate the equipment and obscure the far smaller return from aerodynamically shaped and often much smaller targets such as aircraft. In the near land environment, the large return from the side lobes of the antenna is often sufficient to produce a "ring around" effect, thereby severely limiting the effectiveness of the system even when the main beam of the antenna is pointed at sea.
The prior art has utilized the concept of doppler frequency shift in the separation of the stationary from the moving targets. In these prior art moving target indicator (MTI) systems, the change in doppler frequency due to the speed of the radar platform, the wavelength of the energy transmitted, and the bearing of the reflector measured from the normal to the velocity vector are calculated or detected. These doppler frequencies are then attenuated by the radar signal processor to remove the return from ground targets in the main beam. It is necessary to continually retune the moving target doppler filters to attenuate the ground return doppler frequency bands associated with the antenna direction as the antenna rotates. This tuning operation is commonly referred to as TACCAR.
Stationary target echoes in the main beam of the antenna system have been detected on the basis of their amplitude relative to aircraft echoes as a 60 to 70 db difference in amplitude is comparatively frequent in many areas. Those echoes of sufficient magnitude to override the main beam clutter attenuation are clearly stationary and they are detected and the doppler filters inhibited at that range to prevent a main beam false alarm. This technique is generally known as main beam blanking.
A greater problem occurs with respect to side lobes. Main beam clutter is lower in power and narrower in doppler spectrum as the width of the main beam is narrowed, and main beam clutter can thereby be reduced. However, a reduction in the width of the main beam produces higher side lobes and thus more side lobe clutter.
The magnitude of the ground return signals from stationary targets is often so large relative to the desired moving target signals that the attenuation of the side lobes is also overcome and false alarms are generated. Once a known stationary target has been detected or identified as having sufficient strength to override the side lobe attenuation, the prior art system doppler filters have been inhibited at the calculated range to the target and the false alarm thus prohibited. Inhibiting the doppler filters at a given range unfortunately also precludes the detection of any target at that range, thus seriously reducing the effectiveness of the system.
The advent of increasingly complex and sophisticated offensive and defensive weapon systems and supersonic jet propelled aircraft has vastly increased the requirements of the radar return signal processors for accumulating the received data and for discriminating between false targets and real targets. In so doing, these radar processors have become extremely complex and sensitive to variations in the return signal strength.
Because of these highly sensitive signal processors which are necessary for reliable target discrimination, the magnitude of the difficulties described above have been significantly increased where the search area of the radar includes strong clutter creating area such as foliage, ocean waves and discrete ground reflectors such as buildings. These clutter producing objects can cause wide variations in the clutter signal strength and may produce target-like signals which exceed the dynamic range of the processor.
Prior efforts to reduce the effects of clutter have been directed primarily in the direction of limiting or clamping of the radar return signal to the dynamic range of the radar processors. This solution, however, is unsatisfactory since the limiting of the signal may also obscure much target information in the radar return signals, particularly where the targets are small and airborne.
The problem is especially acute in an airborne moving target indicator (AMTI) system in which the radar platform is itself an airborne aircraft and the doppler search radar is of the look-down type. Weight and physical size also impose practical limitations on the complexity of any system which must be airborne.
The amplitude of the clutter signals in an AMTI system varies widely with range and the foliage backscatter on such a radar return signal has been measured to vary as much as 40 dbs. Discrete objects such as buildings, etc. have been found to produce return signals which vary as much as 70 dbs in excess of adjacent return signals.
It is accordingly a general object of the present invention to minimize and obviate the problems of the prior art system and to provide an improved and novel AEW system having a look-down capability in overland and nearland environments.
It is another object of the present invention to provide a novel system having a reliable air traffic detection capability in a land-clutter environment.
It is still another object of the present invention to provide a novel method and apparatus for false alarm control in an AEW system suitable for automatic data processing equipment.
It is yet another object of the present invention to provide a novel method and apparatus for enhancing the detection range of an AEW radar system.
It is a further object of the present invention to provide a novel method and apparatus for reducing the susceptibility of AEW aircraft moving target indicators to interference and jamming, and to elevate the reliability and increase the ease of maintenance of known AEW moving target indicating systems.
It is yet a further object of the present invention to provide a novel method and apparatus for the automatic detection and tracking of small airborne targets flying over urban terrain and for the suppression of fixed target clutter.
It is another general object of the present invention to provide a novel method and amplitude adjustment system for optimizing target discrimination by a radar processor.
It is another object of the present invention to provide a novel method and automatic gain control system for radar return signal processors which dynamically adjusts the amplitude of the radar return signals to match the dynamic range of the radar processors.
It is still another object of the present invention to provide a novel automatic gain control method and system in conjunction with a wide band amplifier, the gain control system being effective to adjust the gain of the amplifier during the time the return signal is being received.
It is a further object to provide a novel method and system wherein the interpulse period is divided into several hundred or more time intervals each less than a microsecond in duration to thereby effectively divide the radar return signal into range bins, and to control the amplification of the radar return signals in the other range bins. The amplification for each of the range bins is related to the amplitudes of the radar return signals for the corresponding range bins of previous radar return signals.
It is still a further object to provide a novel system including a wide band amplifier and method in which the transients generated by the adjustment of the gain of the cascaded amplification stages are superimposed onto the same portion of the signal being amplified.
Yet a further object is to provide a novel system including an automatic gain control circuit and method in which the discrete gain adjustment of the amplifier for different portions of the signal is responsive, within predetermined limits, solely to the sampled amplitude of the same portion of the return signal which immediately preceded the return signal being amplified.
Yet still a further object is to provide a novel method and system for eliminating moving targets in the radar mapping of overland areas.
Yet still a further object of the present invention is to obviate the need for main beam blanking in MTI radar systems through the use of a novel automatic gain control circuit.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from the claims and from the following detailed description when read in conjunction with the appended drawings.